1. Field of the Invention
This invention relates generally to medical imaging, and particular to an apparatus and method for imaging a medical instrument, particularly while being inserted inside a patient.
2. Description of Related Art
Some medical procedures require a needle or needle-like instrument to be inserted into a patient's body to reach a target. Examples of these procedures include tissue biopsies, drug delivery, drainage of fluids, ablation for cancer treatment, and catheterization. Some of these procedures can be done manually without any additional guidance other than the sense of feel and visualization of the surface of the body. Other procedures are difficult to perform without additional guidance because the target is deep, the target is small, sense of feel is inadequate for recognizing when the needle's tip has reached the target, or there is a lack of visual landmarks on the body surface. In those cases, providing the health care provider with an image of the interior of the body in the vicinity of the target would be beneficial. It would be particularly beneficial to provide real-time images of both the target and the needle as it progresses towards the target.
A particularly challenging needle insertion procedure is required in epidural anesthesia, often referred to as an “epidural” in the field of obstetrics. Epidural anesthesia is administered in the majority (>80% of women in labor) of patients for pain relief of labor and delivery in North American hospitals. Epidural anesthesia involves the insertion of a needle into the epidural space in the spine. The anatomy of the back and spine, in order of increasing depth from the skin, includes the skin and fat layers, a supraspinous and interspinous ligament, the epidural space, the dura mater and spinal cord. A doctor must insert the needle through these layers in order to reach the epidural space without over-inserting the needle and puncturing the thin dura mater surrounding the spinal cord.
The traditional procedure of epidural needle insertion will now be described. The patient is seated with the doctor facing the patient's back. The doctor chooses a puncture site between the vertebrae based on feeling the protruding spinal processes. After choosing an insertion point on the skin, the doctor typically inserts the needle in a plane midline with the long axis of the spine. A saline-filled syringe is attached to the needle so the doctor can apply pressure to the plunger of the syringe, as the needle in incrementally advanced toward the epidural space, and feel how easily saline is injected into the tissue. This is called the “loss-of-resistance”method because resistance falls when the needle tip enters the epidural space. In this way, the sense of feel is the main method for determining when the needle tip has reached the epidural space because the saline is easily injected into the epidural space compared to the tissue encountered before the epidural space. This method can result in failure rates of 6 to 20% depending on the experience and training of the health care provider. Complications include inadvertent dura puncture resulting in loss of cerebral spinal fluid and headache, as well as nerve injury, paralysis and even death. Image guidance during needle insertion would improve the accuracy of needle insertion by providing better feedback to the doctor of where the needle is located with respect to the anatomical structures including the target.
In the past several years, ultrasound has been explored as a means to provide a pre-puncture estimate of the depth of the epidural space to correctly place the needle tip. This entails an ultrasound scan prior to needle insertion so that the doctor uses the knowledge of how deep to expect the epidural space when inserting the needle. This use of ultrasound at the planning stage for epidural guidance has received wide interest from the anesthesia community. It is called pre-puncture scanning because the ultrasound is used before, but not during, needle insertion. The National Institute for Health and Clinical Excellence (NICE) has recently issued full guidance to the NHS in England, Wales, Scotland and Northern Ireland on ultrasound-guided catheterization of the epidural space (January, 2008). While pre-puncture scanning is a useful advance, doctors still face challenges associated with performing needle insertion procedures without information provided by real-time imaging.
There have been some published reports of providing real-time ultrasound imaging for needle insertion procedures. However, none of these approaches have proven to be entirely satisfactory. Problems include overly limiting views of the images of the target and needle due to poor reflection of ultrasound waves, and/or inherent limitations in the ultrasound equipment.